pstore: d_alloc_name() doesn't return an ERR_PTR

d_alloc_name() returns NULL on error.  Also I changed the error code
from -ENOSPC to -ENOMEM to reflect that we were short on RAM not disk
space.
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Tony Luck <tony.luck@intel.com>

fs/pstore/inode.c

@@ -345,9 +345,8 @@ int pstore_mkfile(enum pstore_type_id type, char *psname, u64 id, int count,
 
 	mutex_lock(&root->d_inode->i_mutex);
 
-	rc = -ENOSPC;
 	dentry = d_alloc_name(root, name);
-	if (IS_ERR(dentry))
+	if (!dentry)
 		goto fail_lockedalloc;
 
 	memcpy(private->data, data, size);